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<title>STABS: Stab Section Basics</title>

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<a name="Stab-Section-Basics"></a>
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<p>
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<hr>
<a name="How-to-Embed-Stabs-in-Sections"></a>
<h3 class="appendixsec">F.1 How to Embed Stabs in Sections</h3>

<p>The assembler creates two custom sections, a section named <code>.stab</code>
which contains an array of fixed length structures, one struct per stab,
and a section named <code>.stabstr</code> containing all the variable length
strings that are referenced by stabs in the <code>.stab</code> section.  The
byte order of the stabs binary data depends on the object file format.
For ELF, it matches the byte order of the ELF file itself, as determined
from the <code>EI_DATA</code> field in the <code>e_ident</code> member of the ELF
header.  For SOM, it is always big-endian (is this true??? FIXME).  For
COFF, it matches the byte order of the COFF headers.  The meaning of the
fields is the same as for a.out (see <a href="Symbol-Table-Format.html#Symbol-Table-Format">Symbol Table Format</a>), except
that the <code>n_strx</code> field is relative to the strings for the current
compilation unit (which can be found using the synthetic N_UNDF stab
described below), rather than the entire string table.
</p>
<p>The first stab in the <code>.stab</code> section for each compilation unit is
synthetic, generated entirely by the assembler, with no corresponding
<code>.stab</code> directive as input to the assembler.  This stab contains
the following fields:
</p>
<dl compact="compact">
<dt><code>n_strx</code></dt>
<dd><p>Offset in the <code>.stabstr</code> section to the source filename.
</p>
</dd>
<dt><code>n_type</code></dt>
<dd><p><code>N_UNDF</code>.
</p>
</dd>
<dt><code>n_other</code></dt>
<dd><p>Unused field, always zero.
This may eventually be used to hold overflows from the count in
the <code>n_desc</code> field.
</p>
</dd>
<dt><code>n_desc</code></dt>
<dd><p>Count of upcoming symbols, i.e., the number of remaining stabs for this
source file.
</p>
</dd>
<dt><code>n_value</code></dt>
<dd><p>Size of the string table fragment associated with this source file, in
bytes.
</p></dd>
</dl>

<p>The <code>.stabstr</code> section always starts with a null byte (so that string
offsets of zero reference a null string), followed by random length strings,
each of which is null byte terminated.
</p>
<p>The ELF section header for the <code>.stab</code> section has its
<code>sh_link</code> member set to the section number of the <code>.stabstr</code>
section, and the <code>.stabstr</code> section has its ELF section
header <code>sh_type</code> member set to <code>SHT_STRTAB</code> to mark it as a
string table.  SOM and COFF have no way of linking the sections together
or marking them as string tables.
</p>
<p>For COFF, the <code>.stab</code> and <code>.stabstr</code> sections may be simply
concatenated by the linker.  GDB then uses the <code>n_desc</code> fields to
figure out the extent of the original sections.  Similarly, the
<code>n_value</code> fields of the header symbols are added together in order
to get the actual position of the strings in a desired <code>.stabstr</code>
section.  Although this design obviates any need for the linker to
relocate or otherwise manipulate <code>.stab</code> and <code>.stabstr</code>
sections, it also requires some care to ensure that the offsets are
calculated correctly.  For instance, if the linker were to pad in
between the <code>.stabstr</code> sections before concatenating, then the
offsets to strings in the middle of the executable&rsquo;s <code>.stabstr</code>
section would be wrong.
</p>
<p>The GNU linker is able to optimize stabs information by merging
duplicate strings and removing duplicate header file information
(see <a href="Include-Files.html#Include-Files">Include Files</a>).  When some versions of the GNU linker optimize
stabs in sections, they remove the leading <code>N_UNDF</code> symbol and
arranges for all the <code>n_strx</code> fields to be relative to the start of
the <code>.stabstr</code> section.
</p>
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